In the liquid crystal display device, a TFT substrate has pixel electrodes and thin film transistors (TFTs), and the like, arranged at a matrix form. A color filter substrate has a color filters arranged at positions corresponding to the pixel electrodes of the TFT substrate. The TFT substrate and the color filter substrate are opposite to each other with a liquid crystal interposed therebetween. In such a configuration, the liquid crystal display device forms an image by controlling the transmittance of light of the liquid crystal molecules for each pixel.
Liquid crystal display devices can be made small and thin, and are used in a wide range of applications in mobile phones or other electronic devices. Recently a wide variety of applications have been implemented in mobile phones. There is also a demand for input devices to have a function allowing finger input through a touch panel, in addition to the conventional key-button operation. In this case, the touch panel is mounted on the side of the color filter substrate of the liquid crystal display panel.
Meanwhile, in the liquid crystal display device, there is a strong demand for reducing the thickness of the liquid crystal display panel, in addition to reducing the overall size of the set, while keeping a certain size of the screen. The thickness of the liquid crystal display panel is reduced by polishing the outside of the liquid crystal display panel after production of the liquid crystal display panel. A liquid crystal display panel includes two glass substrates. One is a TFT substrate in which pixel electrodes, thin film transistors (TFTs), and the like, are formed. The other is a color filter substrate in which color filters are formed. The TFT substrate and the color filter substrate are standardized, for example, to 0.5 mm or 0.7 mm. It is difficult to obtain such standardized glass substrates from a market. In addition, very thin glass substrates have a problem relating to mechanical strength and bending in the production process, leading to a reduction in the production yield. As a result, the liquid crystal display panel is formed by the standardized glass substrates, and then the outside of the liquid crystal display panel is polished to reduce the thickness of the liquid crystal display panel.
The reduction of the thickness of the liquid crystal display panel poses a problem of the mechanical strength. When a mechanical pressure is applied to the display surface of the liquid crystal display panel, there is a risk that the liquid crystal display panel will be broken. This is the same for the touch panel set to the liquid crystal display panel due to a small thickness of the touch panel.
In order to prevent the liquid crystal display panel from being broken by outside force, a front window of resin or glass is attached to the screen side of the liquid crystal display panel. In this case, an air layer exists between the liquid crystal display panel and the touch panel, or between the touch panel and the front window. The transmittance of the light from the backlight is reduced by the reflection from the interface in this area.
In order to prevent this, JP-A No. 83491/2008 describes a configuration in which an adhesive layer or an anti-reflection coating is formed between the liquid crystal display panel and the touch panel, or between the touch panel and the front window.
A liquid crystal display device having a liquid display panel, a touch panel, and a front window, is often labeled with a logo representing the abbreviation of the phone function or the telephone service company. Such a logo is formed by printing on the back side of the touch panel.
The logo is displayed in white or a predetermined color. The display is formed by applying a predetermined color or white to a cut-out shape of a predetermined logo printed in black. As a result, the thickness of the recoated portion of the logo is increased to about 20 μm.
The front window and the touch panel are bonded together with an adhesive sheet. If a thick printed mark is formed between the adhesive sheet and the front window, this will have an adverse effect on the adhesive force between the touch panel and the adhesive sheet. Thus, the recoated portion of the logo should be formed on the outside of the adhesive sheet.
A touch-panel flexible wiring substrate is connected to the touch panel in order to supply electric current and signals to the touch panel. For this reason, the adhesive sheet may not be provided in the vicinity of the area in which the touch-panel flexible wiring substrate is mounted.
However, when the recoated logo is formed as described above and is provided on the outside of the adhesive sheet, the touch panel, the touch-panel flexible wiring substrate, and the logo may overlap with each other. The overlapping of the three components may cause a stress peeling apart the front window and the touch panel. Such a stress has an adverse effect on the reliability of the liquid crystal display panel.